These are the convincing risk factors for developing breast cancer (Cancer Australia 2018) (those highlighted in bold are modifiable):

• age
• gender (being female)
• significant family history of breast cancer and/or other cancers
• pathogenic variants in cancer predisposition genes including BRCA1, BRCA2, CDH1, PALB2, PTEN, NF1, STK11, TP53, ATM and CHEK2
• DCIS (ductal carcinoma in situ)
• LCIS (lobular carcinoma in situ) also referred to as non-invasive lobular neoplasia
• atypical epithelial proliferative lesions (atypical ductal hyperplasia and atypical lobular hyperplasia)
• previous breast cancer
• high mammographic breast density (must be adjusted for age and body mass index)
• early menarche
• not bearing children
• never having breastfed
• late age at first birth
• late menopause
• maternal exposure to diethylstilboestrol (DES) in utero
• use of combined hormone replacement therapy, particularly for extended periods over many years
• not engaging in adequate physically active
• overweight and obesity (only for postmenopausal women)
• weight gain (postmenopausal)
• alcohol consumption
• exposure of the breast to ionising radiation.

For more information, visit the Cancer Australia breast cancer risk factor website.